Protonation by Hydronium Ion

As an example of the problem of species in solution, consider the case of a solution made by dissolving some potassium chrome alum, KCrfSO s-12H20, in water. On testing, the solution is distinctly acidic. A currently accepted explanation of the observed acidity is based upon the assumption that, in water solution, chromic ion is associated with six H20 molecules in the complex ion, Cr(H20) a. This complex ion can act as a weak acid, dissociating to give a proton (or hydronium ion). Schematically, the dissociation can be represented as the transfer of a proton from one water molecule in the Cr(H20) 3 complex to a neighboring H20 to form a hydronium ion, H30+. Note that removal of a proton from an H20 bound to a Cr+3 leaves an OH- group at that position. The reaction is reversible and comes to equilibrium ... 

Fig. 5 Logarithmic plots of rate-equilibrium data for the formation and reaction of ring-substituted 1-phenylethyl carbocations X-[6+] in 50/50 (v/v) trifluoroethanol/water at 25°C (data from Table 2). Correlation of first-order rate constants hoh for the addition of water to X-[6+] (Y) and second-order rate constants ( h)so1v for the microscopic reverse specific-acid-catalyzed cleavage of X-[6]-OH to form X-[6+] ( ) with the equilibrium constants KR for nucleophilic addition of water to X-[6+]. Correlation of first-order rate constants kp for deprotonation of X-[6+] ( ) and second-order rate constants ( hW for the microscopic reverse protonation of X-[7] by hydronium ion ( ) with the equilibrium constants Xaik for deprotonation of X-[6+]. The points at which equal rate constants are observed for reaction in the forward and reverse directions (log ATeq = 0) are indicated by arrows.

Monoprotonation of the [2.1.1]-cryptand occurs rapidly but protonation of the monoprotonated species by hydronium ion and other acids can be followed kinetically in various solvents (Cox et al., 1982, 1983). In methanol, protonation of ii+ species by substituted acetic and benzoic acids to give i+i+ has been studied using the stopped flow technique with conductance detection. The values of the rate coefficients (kHA) for protonation (81) vary with the acidity of the donor acid from kHA = 563 dm3mol-1s-1 (for 4-hydroxy-benzoic acid) to kHA = 2.3 x 105 dm3mol 1s 1 (for dichloroacetic acid). 

Prior to 1967 acetal hydrolysis had been found to be a specific-acid catalysed reaction with the accepted mechanism [equation (46)] involving fast pre-equilibrium protonation of the acetal by hydronium ion, followed by unimolecular rate-determining decomposition of the protonated intermediate to an alcohol and a resonance stabilized carbonium ion (Cordes, 1967). An A-1 mechanism was supported by an extremely large body of evidence, but it appeared unlikely that such a mechanism could expledn the... 

The modest amount of scatter in Fig. 10 is remarkable, considering that it includes four different reaction types (carbon protonation of enols or enolates by hydronium ions or by water) and a wide range of substrates. The standard deviation between the 62 observed values of log kK and those calculated by Equation (19) is 0.95. 

Fig, 9. Brdnsted plot for the protonation of aromatic substrates by hydronium ion and for the deprotonation of the benzenonium ions by water. Data are statistically corrected and the numbers correspond to the compounds in Table 10. For a description of the rate coefficients kT (open points) and krev (closed points) see text. Redrawn with permission from A. J. Kresge et al., J, Am. Chem. Soc., 93 (1971) 6181. Copyright by the American Chemical Society. 

H2O serves as a base in 17-3) and as an acid in 17-4). Note that the bare of 17-1) becomes the hydrated proton or hydronium ion, HsO, of 17-3). In the formulation of equilibrium constants, [H ] and [H30 ] are always equivalent to each other the two forms are used interchangeably in most contexts and will be so used in all ionic equilibrium problems in this book. Although the proton is indeed hydrated in aqueous solutions, the notation H is often used instead of H30 because the reader understands the fact of hydration, because he need not worry about specifying the exact extent of hydration (which exceeds one H2O per proton), and because the specific use of the hydrated formula for the proton might obscure the important fact that all ions in water are extensively hydrated. Note also that the denominator in the Ka expression in 17-3) is identical with that in 17-1) since applications of these equilibria are intended for dilute solutions, H2O is always taken to be in its standard state and therefore need not be represented by a term in the expression. Equation 17-4) avoids describing aqueous ammonia as NH4OH, a species that probably does not exist at ambient temperatures. 

The microscopic reverse of deprotonation of a carbon acid by water is protonation of the product carbanion by hydronium ion (Scheme I.IA), with a limiting rate constant of (ka)H 10 s for diffusional encounter of the carbanion and... 

The kinetics of protonation and hydronium-ion catalyzed hydrolysis of alkoxythiophenes, alkoxy-benzo[6]thiophenes, and (alkylthio)benzo[6]thiophenes have been determined by H-NMR spectroscopy <87X69,92T7823>. Thus, when 3-methoxythiophene is dissolved in CD3CN D2O which is 0.1 M in DCl, the H-2 signal disappears completely after 3 min at 32°C, but no hydrolysis takes... 

The mobility of ions in aqueous solution is well described by Stokes law where the limiting mobility is determined by the ion s radius and the viscosity of the solution. Since an hydronium ion has a radius midway between that of a potassium and sodium ion, we might expect its mobility to lie somewhere between 5.8x10 and 7.6 x 10 m V s (Ref. 33, p. 104). In fact, the mobility of protons and hydronium ions in aqueous solution are similar and of a value (36.3 x 10 m s ) considerably greater than that predicted by Stokes law. The key to an understanding of this lies in the observation that proton mobilities in nonpolar liquids such as hexane or propanol are not abnormally larger. Hydrogen-bond networks therefore seem important, and in fact it is the translocation of... 

Because conductivity values and activatitMi enthalpies derived from the first measurement series do not show any systematic dependence on composition nor on the kind of alkali cation, they seem to be determined by an arbitrary amount of residual water. In contrast, the dc conductivity and enthalpy values determined for dried PEC remain unaffected by further heating or cooling processes and show clear trends. The loss of water generally lowers the ionic conductivity of dried PEC by several orders of magnitude compared to as-prepared PEC. In addition to cations or anions, protons or hydronium ions might contribute to the dc conductivity of the non-annealed PEC samples determined in the first heating run. 

The transport reactions of the gastric H,K ATPase. As a function of binding MgATP and H or hydronium (HsO ) ion, the export of protons or hydronium ions occurs after phosphorylation. In the presence of K extracellularly enabled by the presence of a KCI channel in the canalicular membrane, K binds to the outward conformation of the phosphorylated pump. The K is then transported inwardly during the dephosphorylation step. In the absence of K, the pump stops in the E2P conformation. 

The mechanism of enolization involves two separate proton transfer steps rather than a one step process m which a proton jumps from carbon to oxygen It is relatively slow m neutral media The rate of enolization is catalyzed by acids as shown by the mechanism m Figure 18 1 In aqueous acid a hydronium ion transfers a proton to the carbonyl oxygen m step 1 and a water molecule acts as a Brpnsted base to remove a proton from the a car bon atom m step 2 The second step is slower than the first The first step involves proton transfer between oxygens and the second is a proton transfer from carbon to oxygen... 

The mode of extraction in these oxonium systems may be illustrated by considering the ether extraction of iron(III) from strong hydrochloric acid solution. In the aqueous phase chloride ions replace the water molecules coordinated to the Fe3+ ion, yielding the tetrahedral FeCl ion. It is recognised that the hydrated hydronium ion, H30 + (H20)3 or HgO,, normally pairs with the complex halo-anions, but in the presence of the organic solvent, solvent molecules enter the aqueous phase and compete with water for positions in the solvation shell of the proton. On this basis the primary species extracted into the ether (R20) phase is considered to be [H30(R20)3, FeCl ] although aggregation of this species may occur in solvents of low dielectric constant. 

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